1. Field of the Invention
The present invention relates to a motor driving device and a vacuum pump having the motor driving device.
2. Description of the Related Art
In vacuum pumps such as turbo-molecular pumps for rotating rotors at a high speed so as to carry out evacuation, DC brushless motors are frequently used as motors for driving to rotate rotors. In vacuum pumps in which rotation sensors are not used, rotational speed information necessary for rotation driving and magnetic pole position information of motor rotors are estimated based on detection signals relating to three-phase voltages and three-phase currents of motors (for example, JP 8-256496 A).
Conventionally, as a method for estimating a magnetic pole position, an estimated electrical angle θ is obtained according to arc tangent operation θ=atan(−E/Eβ). However, two-phase ac signals Eα and Eβ include harmonics (harmonics noises such as PWM carrier components) as well as rotational component fundamental harmonics. For this reason, a fluctuation in a waveform of the calculated estimated electrical angle θ is great at every period, a steady-state error occurs, and accuracy of a magnetic pole position cannot be improved
On the other hand, a rotational speed ω is obtained according to ω=√(Eα^2+Eβ^2)/k by utilizing a state that a counter electromotive voltage is proportional to the rotational speed. Since the rotational speed ω is calculated directly based on the two-phase ac signals Eα and Eβ, similarly it is affected by noises. The calculation of an electrical angle θ utilizes periodicity, but the calculation of the rotational speed ω does not utilize periodicity. For this reason, an error of an estimated amplitude value of the counter electromotive voltage directly becomes a steady-state error.
In the conventional technique, since a fluctuation of an electrical angle (namely, a fluctuation in a magnetic pole position) is great, a ripple occurs in a motor current, and thus driving stability is not satisfactory. Further, electrical efficiency is not satisfactory.